The invention relates generally to an electrical switch having a precise actuation position for sensing a position of an engaging member. The invention relates particularly to an electrical switch for sensing when tape is about to end in a reversing video tape recording apparatus. While the particular embodiment of the invention described herein is designed to be utilized in a high-speed reversing video tape recording apparatus, it is to be understood that the present invention may be advantageously utilized in any apparatus wherein it is desirable to precisely sense the position of an engaging movable member.
There are numerous apparatus in which the operation of the apparatus is controlled based upon a position of one or more members of the apparatus. This control is often provided by electric circuitry which is actuated by the member engaging an actuator of a snap-action-plunger or leaf-action miniature switch, generally known as microswitch. An example of such an apparatus is shown in U.S. Pat. No. 3,958,272, issued in the name of Gerhard Rotter et al., and entitled "Turn Around Method and Circuit". The micro-switches (referred to as 17 and 18) are used in a tape transport apparatus to sense the position of carriages which are movable in the direction of a capstan.
The details of a typical tape transport apparatus are shown in U.S. Pat. No. 4,023,748, issued in the name of D. L. Burdorf et al., and entitled, "Cassette Loading and Tape Tensioning System". The apparatus described therein includes a pair of movable pivot arms, each of which is pivoted at one end and carries a journaled reel of tape at the other end. The reels are urged by a spring force into compressive surface engagement with a capstan, whereby the recording tape is transferred from one reel, past a record/playback head, to the other reel. As the tape is so transfered, the diameters of the reels change accordingly and the pivot arms accomodate the change by pivoting to maintain the outer layer of tape on each reel in contact with the capstan. The movement of the pivot arms towards the capstan can be precisely used to determine the diameter of tape on the reel and be correlated to sense the end of tape of the respective reel by the engagement of the pivot arms into a suitable switch.
A problem with prior art switches for tape turn around in video recorder apparatus is the variation in actuation position of the switch. In some arrangements, the pivot arms can be extended beyond the hub of the reel to contact a micro-switch, and thereby magnify the travel of the arm and magnify the sensitivity of the actuation of the switch relative to the diameter of the hub. However, a particular problem exists due to confinement and interference of components which requires that the switch be located between the pivot point and the hub of the lever arm which thereby diminishes the travel of the member and decreases the sensitivity of the switch relative to the diameter of the hub. On a tape system driven at 4 meters per second, any variability in actuation creates a significant range of tape travel that must be monitored in order to precisely locate the point of turn around for the system. A magnifying lever arm arrangement may have been acceptable to operate with a switch having an actuation precision of .+-.0.001 inches; however, a confined lever arm arrangement may require a switch having an actuation precision of .+-.0.00025 inches for the same sensing performance. Heretofore, no such precision switch was commercially available.
Another problem with tape turn around switches is that upon engagement of the carriage with the switch, the carriage continues pivoting on its axis until the circuitry actually changes direction on the tape. This overtravel has normally been absorbed by elastically mounting the switch. Elastic mounting of the switch usually contributes to variability in the actuation of the switch. The overtravel requirement of the switch has also been attempted to be solved by using an elastic actuator such as a freely deflectable conductive spring. However, a problem is created in that the engagement force is not always sufficient for electrical conduction and results in variability of the actuation position of the switch. This problem is particularly acute where the switch is controlling a circuit having low current of approximately 2 milliamps at 5 volts such as in the video tape recording example.
Still another problem with switches of the prior art is that the calibration and adjustment is difficult and the entire switch must be repositioned to adjust the actuation position to correspond with the engagement of the member of the apparatus.